1. Field of the Invention
The invention relates generally to the field of monitoring the composition of gases and, more particularly, to solid state devices incorporating palladium (Pd) metal films, and methods relating thereto for measuring hydrogen concentration in a gas composition.
2. Discussion of the Related Art
Hydrogen sensors are useful for determining the relative amount of hydrogen in an atmosphere of interest. A typical hydrogen sensor functions based on the fact that the electrical properties of a number of palladium containing compositions vary as a function of their hydrogen content, the hydrogen content of the composition being in-turn a function of the partial pressure of hydrogen in the surrounding atmosphere. U.S. Pat. No. 5,338,708 to Felten, entitled "Palladium Thick-Film Conductor", describes compositions useful for hydrogen sensors.
U.S. Pat. No. 5,451,920 to Hoffheins et al. describes a thick film hydrogen sensor element which includes an essentially inert, electrically-insulating substrate having deposited thereon a thick film metallization forming at least two resistors. The metallization is a sintered composition of Pd and a sinterable binder such as glass frit. An essentially inert, electrically insulating, hydrogen impermeable passivation layer covers at least one of the resistors.
U.S. Pat. No. 5,367,283 to Lauf, et al. describes a thin film hydrogen sensor element which includes an essentially inert, electrically-insulating substrate; a thin-film metallization deposited on the substrate, the metallization forming at least two resistors on the substrate, the metallization including a layer of Pd or a Pd alloy for sensing hydrogen and an underlying intermediate metal layer for providing enhanced adhesion of the metallization to the substrate; and an essentially inert, electrically insulating, hydrogen impermeable passivation layer covering at least one of the resistors.
Referring to FIG. 1, a hydrogen sensor 10 made in accordance with U.S. Pat. Nos. 5,367,283 and 5,451,920 is shown. A nonconductive substrate 11 is provided with four conductive pads 12 deposited by thick-film metallization or other suitable technique. These pads 12 serve as a structure for interconnecting the sensor to measurement electronics, not shown. Four conductive metallizations 13, 14 of Pd or a Pd alloy are deposited between the pads 12 and form the four elements of a Wheatstone bridge circuit. Two of these conductive metallizations 13 are exposed to the surrounding atmosphere and the other two metallizations 14 are covered by a dense, hydrogen impermeable coating 15. When hydrogen is present in the gas surrounding hydrogen sensor 10, some hydrogen dissolves in the "active" metallizations 13 and their electrical resistance increases relative to that of the "reference" metallizations 14, which are prevented from absorbing hydrogen by the coating 15. The resistance increase in the "active" metallizations 13 causes an imbalance in a Wheatstone bridge circuit. The imbalance is directly related to the hydrogen concentration.
Previously disclosed hydrogen sensors are limited to certain ranges of hydrogen concentrations for optimal operation because of the well-known phenomenon that affects all Pd-based sensors at very high hydrogen concentrations, viz., the formation of a Pd hydride phase and the stresses associated with the corresponding volume change. In more detail, after exposure to high hydrogen concentrations, or repeated exposures to intermediate hydrogen concentrations, gradual delamination of the hydride forming "active" metallization from an underlying ceramic substrate can occur. This renders the sensor unreliable and can lead to total failure by open circuit of the associated Wheatstone bridge circuit. Making the metallization more adherent normally involves diminished sensitivity.
One previously proposed solution to this problem is to use a Pd alloy instead of pure Pd. However, the solubility of H in Pd alloys is lower than in pure Pd, and the electrical resistance of the alloy is higher than that of the pure metal. The inherent sensitivity of the resistive sensor is proportional to .DELTA.R/R.sub.0, so with regard to a Pd alloy, these two effects (lower .DELTA.R, higher R.sub.0) conspire to reduce the overall sensitivity of an alloy-based sensor relative to that of a pure Pd-based device.
Another previously proposed solution is to reformulate the paste used to form the metallizations 13 and 14 by increasing the proportion of glass frit and decreasing the proportion of Pd. It can be appreciated that this approach will have the same drawbacks (lower .DELTA.R, higher R.sub.0) as discussed in the previous case of alloying.
Heretofore, the requirements of reduced delamination and breakage without reduced sensitivity have not been fully met. What is needed is a solution that addresses all of these requirements simultaneously. The invention is directed to meeting these requirement, among others.